This invention relates to a recording medium especially adapted for electrophotographic copiers and printers, and to a method for making the same.
Black and color electrophotographic copiers and printers utilize dry toner to form an image on coated and usually calendered paper. As is known, dry toner as used in a copying machine is electrostatically adhered to paper and then heated and fused to the paper. Fusion is the heating and melting of toner on the paper to cause the toner to become attached to and stay on the paper. To fuse the toner, either one or both sides of the paper sheet may be heated.
To obtain good quality copies from an electrophotographic process, the dry toner electrostatically adhered to the paper must be properly heated and fused on the paper. This requires that adequate heat be applied to the toner and paper to cause complete melting of the toner on the paper so that the toner stays on the paper and a good image is obtained. However, the application of too much heat can damage the paper and degrade the resulting image. On the other hand, if too little heat is applied, the toner may not be properly heated and melted and some of the toner may come off of the paper, degrading the quality of the resulting image.
Another cause of image degradation in the electrophotographic process is bubbling. This problem is associated with running coated paper in a high temperature electrophotographic printer. Although the exact cause of bubbling is not known, it is believed that when coated paper is heated to fuse the toner to the surface, the coating acts as a moisture sealant on the base media. When the base media is heated, it is thought that moisture trapped within the base media is vaporized by the heat but is unable to escape due to the coating on opposite sides of the base media, resulting in localized bubbling in the coating. The ability to properly fuse toner on paper at a lower temperature alleviates the bubbling pattern.
Another problem encountered in the electrophotographic process has to do with runnability of paper sheet through a copier. Runnability refers to the ability of the paper to feed and deliver through a copying machine without causing jams. The runnability of paper is influenced by the xe2x80x9ctoothxe2x80x9d of its surface, such that a paper surface with more tooth provides for more friction between it and drive rollers of the copier, resulting in improved runnability of the paper through the copier. Obviously, it is not acceptable for paper to excessively jam within a copier. There is even a greater problem when heretofore papers of even very heavy weight, called cover weight papers were used. They are called cover weights, because typically they might be used to form the front and rear covers of a report. Here, unless at very slow speeds, it was difficult to fuse toner to such papers.
The present invention provides an improved coated paper that is adapted to improve fusion of toner on a surface thereof at marginal fusion temperatures in an electrophotographic process, together with an apparatus for making the paper, a process for making the paper and use of the paper in an electrophotographic process. The coated paper itself comprises a paper base and a coating on each side of the base, with the coating being of a weight on the order of 2.5-5.5 pounds per side per ream. Advantageously, the coating has a weight on the order of 3.5-4.0 pounds per side per ream, which is a very light coat weight as compared with the coat weight found on conventional coated gloss paper, and the coated paper has a basis weight of at least 60 pounds per ream. As a result of the light coat weight, the paper of the invention has a coating weight/basis weight ratio that is on the order of 33%-59% of the coating weight/basis weight ratio of conventional coated gloss paper for a given basis weight. Also as a result of the very light coat weight, the paper of the invention has a coating weight/fiber weight ratio that is on the order of 27%-52% of the coating weight/fiber weight ratio of conventional coated gloss paper for a given basis weight.
It is contemplated that paper embodying the invention be made to have a basis weight in the range of about 60-110 pounds per ream. For a basis weight of about 60 pounds per ream, the paper has a coating weight/basis weight ratio on the order of 8%-18%. For a basis weight of about 70 pounds per ream, the coating weight/basis weight ratio is on the order of 7%-16%, and for a basis weight of about 80 pounds per ream, the coating weight/basis weight ratio is on the order of 6%-14%. For a basis weight of about 110 pounds per ream, the coating weight/basis weight ratio is on the order of 4%-10%.
The coated paper of the invention advantageously is formed square on a paper machine so that it has similar properties in both the machine and cross machine directions, and the paper base of the paper contains on the order of 7%-15% filler, which is on the order of 40%-50% less filler than is contained in a conventional paper base. The paper base may be precoated with lightly pigmented coating on each of its sides, with the precoat being at a weight on the order of 1-2.5 pounds per side per ream and the coating being applied on top of the precoat. After being coated and dried, the coated paper is calendered to a 75xc2x0 TAPPI gloss of 35-50, and preferably to a gloss of about 45. To maintain whiteness of the paper, the furnish used in making the paper base advantageously is groundwood free high brightness bleached pulp. It has been learned that the above invention is also useful in producing cover weight copier paper having improved fusion performance, that is, very heavy weight papers which can be used as covers (outer sheets) on reports. Such papers are normally in the range of 110-256 pounds per ream on a text weight basis (60# to 140# cover weight basis).
Heretofore, covers of conventional design made in this weight range were not totally satisfactory for this use, as the toner did not fuse well or uniformly to the high basis weight sheet. However, it has been learned that producing a high basis weight sheet by using high fiber weight and low coat weight, the toner retention and fusion can be improved. It has been further learned that by taking steps to keep or increase bulk, including in the base sheet, toner retention and fusion can be improved. It is believed that maintained or increased bulk by maintaining or increasing voids in the base sheet as insulation helps retain the toner heat on the surface where it can cause the toner to fuse, and slows down the heat transfer from the surface into the interior of the base sheet, wherein the base sheet of high weight could otherwise act as a heat sink or heat conductor. Coupling the low coat weight to high fiber content base sheet, particularly where bulk is preserved or increased, permits superior fusion of toner in even very high weight cover sheets and may allow increasing the printing speed. Manners of preserving or increasing bulk such as less supercalendering pressure, applying less coating, hot soft calendering instead of supercalendering, brush calendering, making the coating itself less conductive or with more voids, such as by using plastic pigment, for example, such as hollow sphere plastic pigments, are useful and can result in better toner fusion.